The invention relates in general to measurements for determining at least one characteristic of oil well effluent constituted by multiphase fluid which typically comprises three phases: two liquid phases, crude oil and water, plus a hydrocarbon gas phase. In particular, the characteristics in question are the proportions of the component phases, including the water content of the liquid phase, and the flow-rate values--the total flow rate and the flow rates of the various phases.
In the oil industry, the traditional practice is to separate the effluent into its component phases and to perform measurements on the phases separated in this way. However that technique requires separators to be installed on site, which separators are bulky and expensive items of equipment, and when testing wells, it also requires additional pipes to be put into place.
Numerous proposals have been put forward for developing techniques that would make it possible to avoid using such separators. A description of these developments is to be found in SPE publication 28515 (SPE Annual Technical Conference, New Orleans, Sep. 25-28, 1994) by J. Williams, "Status of multiphase flow measurement research".
Amongst such proposals, U.S. Pat. No. 4,788,852 describes apparatus including a device for measuring gamma ray attenuation, that device being associated with a Venturi total flow rate sensor and being situated at the constriction of the Venturi. Apparatus of that type is also described in patent application WO 94/25859 and in SPE publication 36593 dated Oct. 6, 1996, "Multiphase flow measurement using multiple energy gamma ray absorption (MEGRA) composition measurement" by A. M. Scheers and W. F. J. Slijkerman.